Various types of sensing devices that sense physiological characteristics are currently available. Reflectance-based sensing devices that detect emitted waves which are reflected off the surface of a living tissue in order to obtain a physiological measurement may be particularly useful for detection of certain physiological characteristics.
For example, reflectance-based sensing devices may be used to detect photoplethysmography (PPG) signals. PPG is an optical measurement technique used to detect blood volume changes in the microvascular bed of living tissue, typically by detecting light transmitted through the ear lobe or fingertip. As arterial pulsations enter the capillary bed, changes in the volume of the blood vessels or characteristics of the blood itself modify the optical properties of the capillary bed. The PPG signal is used to measure saturation of peripheral oxygen (SpO2), which is an estimation of the level of oxygen saturation in a fluid, such as blood. The PPG signal can also be used to measure blood pressure.
The majority of PPG devices currently available, rely on simple thresholding, or peak detection algorithms, to find the principal peaks in a detected signal. However, these methods are unreliable when the detected signal is less than ideal. Particular problems may be encountered when the baseline of the AC signal component becomes noisy or complex, as can occur even with mild movement artifacts. For many reflectance-based PPG devices, the majority of the noise may also arise due to direct transmittance of waves from the wave source to the waves detector (i.e. non-reflected waves).
In view of the above, there exists a need for a sensing device that overcomes or ameliorates at least some of the above mentioned problems. There is also a need to provide a reflectance-based sensing device which ensures that its wave detector is effectively shielded from the wave emitting source to reduce noise in the readings.